The present invention relates to a method and to apparatus for thermodynamic analysis of a mixture of fluids, in particular of hydrocarbons.
More particularly, the invention relates to such a method comprising the steps consisting in:
causing the volume of a sample of a given quantity of said mixture to vary step by step in monotonic manner; and
reading the pressure of the sample at each step;
in which method the sample is stirred on each step so as to hasten the mixture reaching thermodynamic equilibrium.
A fundamental operation when characterizing the properties of a fluid extracted from a hydrocarbon deposit lies in determining the saturation point of the fluid. It is this saturation point that determines the conditions under which the fluid will be present as a single phase or as a plurality of phases, i.e. whether it will be in the form solely of a gas or of a liquid, or whether it will be in the form of a mixture of gas and of liquid, and it is essential to know these conditions in order to be able to work the deposit.
The saturation point of a fluid at a given temperature is generally defined as being the pressure at which the single phase fluid begins to be present in the form of a plurality of phases.
Thus, the pressure at which a fluid initially contained in the liquid state in a deposit releases a first bubble of gas as its pressure is decreased is known as the xe2x80x9cbubble pointxe2x80x9d. Conversely, if the fluid is initially in the gaseous state, and its pressure is decreased, the pressure at which the first drop of liquid forms is called the xe2x80x9cdew pointxe2x80x9d.
The saturation point of a given fluid is determined by performing pressure-volume-temperature (PVT) analysis or xe2x80x9cthermodynamicxe2x80x9d analysis, consisting in observing as a function of time the pressure and the change in the volume of a sample of fluid that is subjected to varying pressure. This operation is preferably performed at constant temperature equal to that of the reservoir from which the fluid under investigation was taken.
For this purpose, the fluid is inserted in the single phase state into a sealed enclosure at high pressure known as a xe2x80x9cPVT cellxe2x80x9d which can be maintained in stable manner at the desired temperature. Such a cell is made in conventional manner in the form of a chamber subdivided into two portions by a moving piston. The sample of fluid is placed on one side of the piston and a control fluid is placed on the other side.
When determining the bubble point, the pressure in the cell is then decreased in steps by displacing the moving piston by progressively withdrawing control fluid from the portion of the chamber containing it. The sample fluid can thus expand and occupy a larger volume. During this process, the pressure P and the change in volume xcex94V are observed as a function of time t, and a curve of P as a function of xcex94V is plotted.
When the pressure reaches the saturation point, the fluid begins to release the second phase, in this case the gas phase, on the fluid reaching the bubble point. This point is detected by a discontinuity in the slope of the curve
P=f(xcex94V)
FIGS. 1 and 2 show an example of the method as outlined above.
FIG. 1 is a plot at constant temperature showing how volume and pressure varied as a function of time (in seconds) during an experiment seeking to determine the bubble point of a mixture of liquid hydrocarbons.
Curve 1 shows the variation in the volume (in cm3) of the sample, and curve 2 shows the variation of its pressure (in MPa). The periods 3 during which the mixture is in thermodynamic equilibrium can be seen clearly in the form of level portions of stable pressure. The periods 4 during which thermodynamic equilibrium is changing can also be seen clearly in this figure.
In FIG. 2, the time parameter has been eliminated so that pressure is shown directly as a function of change in volume. This representation is constituted in the present case by two half-curves 5 and 6 in which respectively volume and pressure are relatively constant.
The saturation point (Psat) appears as the point of the FIG. 2 curve where it presents a discontinuity of slope.
During the manipulation that leads to the curves of FIGS. 1 and 2, it is necessary at each step to wait for pressure to stabilize (period 4). The accuracy with which the saturation point is determined depends to a very large extent on the quality of the thermodynamic equilibrium.
In order to hasten equilibrium, it is known that the fluid contained in the PVT cell can be stirred. For this purpose, the cell is rotated about an axis and a metal element, such as a bead or a ring, is placed so as to be free to move in the fluid that is to be stirred while the cell is being rotated.
Use has also been made of mercury simultaneously as piston and as stirrer. However, that practice has disappeared because mercury is potentially dangerous for the users implementing the method.
Furthermore, the experiment needs to be repeated several times in order to be sure of the result. It is therefore necessary after the fluid has become dissociated to recombine its two phases by increasing the pressure in the cell beyond the bubble point. In that case also, stirring is necessary in order to hasten recombination.
It is therefore essential for the stirring process to be as effective as possible in order to reduce the time required to reach thermodynamic equilibrium after each step of the experiment. Unfortunately, the known methods mentioned above are of relatively limited effectiveness, only.
The present invention seeks to mitigate that drawback.
More particularly, an object of the invention is to provide a method and apparatus for determining the saturation point of a fluid mixture while reducing the time required for the experiment, and which is independent of the type of hydrocarbon concerned.
To this end, in a first aspect, the invention provides a method of thermodynamically analyzing a mixture of fluids, in particular of hydrocarbons, the method comprising the steps consisting in:
causing the volume of a sample of a given quantity of said mixture to vary step by step in monotonic manner; and
reading the pressure of the sample at each step;
in which method the sample is stirred on each step so as to hasten the mixture reaching thermodynamic equilibrium;
the method being characterized by the fact that the stirring of the sample comprises a step of applying ultrasound to the sample.
It has been found that it is possible to obtain good uniformity of the fluid sample by applying ultrasound thereto. The pressure waves generated in this way assist in bringing the fluid to thermodynamic equilibrium. They avoid metastable equilibrium being established, thus making it possible for the saturation point to be identified with certainty and to be determined with accuracy.
In a first implementation of the method, the ultrasound is applied to a rigid container containing the sample. This applies, for example, when the method is used in a piston device of the type outlined above.
Nevertheless, in another implementation, the ultrasound is applied to a fluid in which a bag of flexible material containing the sample is immersed. This implementation requires a smaller amount of energy.
In a particular implementation, the ultrasound is applied after a succession of steps in which the volume is varied monotonically and during a step in which the direction of volume variation is reversed.
This applies when the measurement is repeated several times and where the pressure therefore needs to be returned to its initial value before beginning a new series of stages in which pressure is varied step by step.
Naturally, the method of the invention can be applied equally well to determining the bubble point of a mixture of fluids in which the pressure of a liquid sample is reduced step by step, or to determining the dew point of a mixture of fluids in which the pressure of a gaseous sample is decreased step by step.
In a second aspect, the invention provides apparatus for determining the saturation point of a mixture of fluids, the apparatus comprising an enclosure suitable for containing a sample of said mixture and means for stirring said sample, the apparatus being characterized by the fact that said stirring means comprises means for generating ultrasound in said sample.
In a first embodiment, said enclosure comprises a rigid container organized to contain directly a sample of said mixture, said means for generating ultrasound being mounted to apply the ultrasound to said container.
In which case, said means for generating ultrasound is disposed on the outside of said container.
In another embodiment, said enclosure comprises a bag of flexible material suitable for containing said sample, said bag being received in a rigid container with a second fluid being interposed between the bag container, said means for generating ultrasound being mounted to apply the ultrasound to said second fluid.
In which case, said means for generating ultrasound is disposed inside said container, immersed in said second fluid.